1. Field of the Invention
The invention is directed to submersible centrifugal pumps and to methods and apparatus for providing pressurized sealing of the motors thereby improving their operational life. The invention is particularly applicable and directed to coker maze pumps used in delayed coker processes to recirculate the water from coker maze pits containing coke fines suspended in water, also known as coke-laden fines. The invention may also have application in other submersible pump services, such as water lift stations and sewage treatment plants.
2. Description of Related Art
Prior art submersible pumps employ a sealed, partially oil-filled chamber having a moisture-sensing probe extending into the oil to detect the presence of any pumpage, which may be water or water containing suspended coker fines. If contamination is detected, steps may be taken to protect the motor. Two seals are utilized on the motor shaft, a lower seal located between the pump and the oil chamber (pumpage-seal) and an upper seal located between the oil chamber and the motor (motor-seal).
Prior art submersible pumps use different seal approaches but do not pressurize the seal chamber. One approach employs stacked lip seals. The lip seals run against the shaft and, over time, will fail. The advantage to this approach is that there is some predictability in timing, which allows for scheduled pump maintenance. Another approach is a non-pressurized mechanical seal. Mechanical seals are the norm in the industrial pump industry as they are generally long lasting, but there is no predictability as to their failure. This is generally not a major problem with a non-submersible pump, but with a submersible pump the pumpage often gets into the motor resulting in electrically shorting out the motor.
Prior art submersible motors generally are not designed to withstand internally generated pressure caused by motor winding temperature increase and/or the temperature rise due to the temperature of the pumpage in the sump/storage tank. The motors depend on pressure release via the non-pressurized motor and pump seals to release the generated pressure. If that avenue is not available, pressure is relieved via electrical or instrument fittings, directly into the water pumpage. When the motor shuts down and cools off, the process is reversed with water leaking into the motor. The motor is shorted out and will fail on the next start
The improvements disclosed herein combine a pressurized oil accumulator in combination with the pump oil chamber described above, and a motor pressure control system.
Pressurized Seal Arrangement
The pressurized oil accumulator in combination with a pump having a submersible motor and pump seals as described herein, can withstand water leakage into the pump and pump motor through the pump and motor mechanical seals. The accumulator is a steel pressure vessel containing a rubber oil bladder. The oil bladder is connected to the reservoir of the pump and is filled with oil for providing make-up oil to the reservoir and for accepting surplus (expansion) oil from the reservoir. The steel shell of the accumulator is pressurized with nitrogen to a pressure higher than the pressure of the pumpage at the pumpage-seal face. The applied pressure in the accumulator pressurizes the pump oil chamber to a pressure equal to the nitrogen pressure in the second compartment. The invention provides the following advantages:
Oil Lubrication of the Seal Faces: Oil pressure in the oil reservoir is maintained higher than the water (pumpage) pressure, therefore the pumpage seal faces are always lubricated by oil, rather than by contaminated water, and any leakage past the seal is from oil to water (pumpage) and not from water to oil. This ensures that water does not migrate into the oil reservoir and into the motor.
Totally Full Oil Chamber: In the prior art, it was necessary to leave expansion room in the oil chamber for oil expansion due to the temperature rise. The improvement of the accumulator tank disclosed herein provides the fluid capacitance to allow for this pressure rise. The oil reservoir is totally filled at all times, keeping the top (motor) seal constantly lubricated.
Accumulator Tank Allows for Pressure Rises: As the pump warms up to operating temperature in the present invention, the bladder in the oil accumulator provides the capacitance to absorb the pressure increase without significantly increasing pressure at the seal faces.
Pressure Pre-Alarm: A pressure switch or transmitter may be installed in the oil chamber to provide a xe2x80x9clow pressurexe2x80x9d pre-alarm to indicate a fall in oil pressure. This is in addition to the moisture probe that is part of the prior art pump. With this pre-alarm, maintenance is limited to a recharge of the oil system, vs. a complete tear-down of the pump which is ultimately required when water gets into the pump oil chamber.